Is there a way that I would be able to create a selectable custom mode similar to the feeding and water changing modes? If say I wanted to create a mode to fill my salt mixing bucket with ro/di water, which would turn on/off a certain relay after a set time. Being able to select it in the menu allows me to initiate the mode at any time.

You can use it like Lee did.Just use a virtual relay port to trigger the mode.Add a second or third relay box, even though you don't physically have it and use the relays on that port to trigger the mode.